Microelectromechanical membrane transducer with active damper

ABSTRACT

A microelectromechanical membrane transducer includes: a supporting structure; a cavity formed in the supporting structure; a membrane coupled to the supporting structure so as to cover the cavity on one side; a cantilever damper, which is fixed to the supporting structure around the perimeter of the membrane and extends towards the inside of the membrane at a distance from the membrane; and a damper piezoelectric actuator set on the cantilever damper and configured so as to bend the cantilever damper towards the membrane in response to an electrical actuation signal.

BACKGROUND Technical Field

The present disclosure relates to a microelectromechanical membranetransducer, to a method for controlling a microelectromechanicalmembrane transducer, and to a process for manufacturing amicroelectromechanical membrane transducer.

Description of the Related Art

As is known, microelectromechanical membrane transducers may be used invarious sectors both for receiving and for generating pressure waves.Microelectromechanical transducers of this type may be used in aunidirectional way (for example, pressure sensors and microphones) orelse in a bidirectional way, for example for providing ultrasound probesfor various applications.

A microelectromechanical ultrasound transducer in general comprises asupporting structure of semiconductor material, formed in which is acavity, and a membrane, which is also of semiconductor material, whichcloses the cavity on one side. A piezoelectric plate is formed on themembrane and is connected to a driving device that alternatively enablesapplication of driving signals to the piezoelectric plate for generatingpressure waves and for detecting oscillations of the membrane caused byreturn echoes of the pressure waves transmitted. In practice, themicroelectromechanical transducer switches between a transmitting modeand a receiving mode. In the transmitting mode, the driving deviceexcites the piezoelectric plate with a pulse train and causes vibrationof the membrane, which produces pressure waves of controlled amplitudeand frequency. In the receiving mode, the membrane is set in vibrationby return echoes caused by variations in the density of the medium alongthe propagation path of the pressure waves emitted in the transmissionstep. The piezoelectric plate converts the oscillations of the membraneinto a transduction signal, which is detected and amplified by thedriving device.

A general problem of microelectromechanical membrane transducers lies inthe time of damping of the oscillations of the membrane upon switchingbetween the transmitting mode and the receiving mode, especially inapplications “in air.”

When the microelectromechanical transducer switches to the receivingmode, the membrane continues to vibrate (i.e., ring) and to produce atail of pressure waves for a time determined by the damping factor,which in general is relatively low in order to obtain a highersensitivity. On the one hand, the tail of pressure waves causes noiseand may render processing of the return signals more complex. On theother hand, it is necessary to wait for the tail of pressure waves tovanish before the microelectromechanical transducer is ready to receivethe return echoes in the receiving mode. This has repercussions on thedepth of the blind area that cannot be investigated because the returnechoes of possible obstacles present would be superimposed on to thetails of the pressure waves.

Solutions have been proposed, which, however, are not satisfactory aboveall because they are excessively sensitive to the variability in themanufacturing processes.

For instance, active suppression envisages detection of the oscillationsof the membrane and application of driving signals in phase opposition,whereas filtering techniques during post-processing aim at eliminatingthe effects of the tails of pressure waves downstream of detection. Inboth cases, however, the effectiveness of the corrective actions is thuslinked to the parameters of the individual microelectromechanicaltransducer and to the environmental conditions, which the unremovablespread of the manufacturing processes would force to calibrate and testeach individual element produced, with consequent unsustainable costs.

BRIEF SUMMARY

The present disclosure provides a microelectromechanical membranetransducer, a method for controlling a microelectromechanical membranetransducer, and a process for manufacturing a microelectromechanicalmembrane transducer, which overcomes or mitigates, among others, thelimitations described above.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the disclosure, some embodiments thereofwill now be described, purely by way of non-limiting example and withreference to the attached drawings, wherein:

FIG. 1 is a partially sectioned perspective view of amicroelectromechanical membrane transducer according to an embodiment ofthe present disclosure;

FIG. 2 is a cross-sectional view through the microelectromechanicalmembrane transducer represented in FIG. 1 ;

FIG. 3 is a top view, sectioned along the line III-III of FIG. 2 andwith parts removed for clarity, of the microelectromechanical membranetransducer of FIG. 1 ;

FIG. 4 shows an enlarged detail of FIG. 2 ;

FIG. 5 a is a graph showing a first quantity regarding themicroelectromechanical membrane transducer of FIG. 1 , in a firstoperating condition;

FIG. 5 b is a graph showing the first quantity, in a second operatingcondition;

FIG. 5 c is a graph showing a second quantity regarding themicroelectromechanical membrane transducer of FIG. 1 , in the firstoperating condition;

FIG. 5 d is a graph showing the second quantity, in the second operatingcondition;

FIG. 6 a is a graph showing a first control quantity used in themicroelectromechanical membrane transducer of FIG. 1 ;

FIG. 6 b is a graph showing a second control quantity used in themicroelectromechanical membrane transducer of FIG. 1 ;

FIG. 7 is a top view, with parts removed for clarity, of amicroelectromechanical membrane transducer according to a differentembodiment of the present disclosure;

FIG. 8 is a top view, with parts removed for clarity, of amicroelectromechanical membrane transducer according to a furtherembodiment of the present disclosure;

FIG. 9 is a top view, with parts removed for clarity, of amicroelectromechanical membrane transducer according to a furtherembodiment of the present disclosure; and

FIGS. 10-14 are cross-sectional views through semiconductor wafers indifferent processing steps of a process for manufacturing amicroelectromechanical membrane transducer according to an embodiment ofthe present disclosure.

DETAILED DESCRIPTION

With reference to FIG. 1 , number 1 designates as a whole amicroelectromechanical membrane transducer according to an embodiment ofthe present disclosure. The microelectromechanical membrane transducer 1comprises a supporting structure 2, within which a cavity 3 is formed, amembrane 5, coupled to the supporting structure 2 so as to cover thecavity 3 on one side, and a cantilever damper 7.

The microelectromechanical membrane transducer 1 further comprises amembrane piezoelectric actuator 8, a damper piezoelectric actuator 10,and a driving device 12 coupled to the membrane piezoelectric actuator 8and to the damper piezoelectric actuator 10. In one embodiment (notillustrated), respective distinct driving circuits can be coupled to themembrane piezoelectric actuator 8 and to the damper piezoelectricactuator 10.

The supporting structure 2 is obtained from a substrate of semiconductormaterial and has, by way of non-limiting example, an annular shape. Thecavity 3 extends through the entire supporting structure 2, is open onone side, and is covered by the membrane 5 on the other side.

The membrane 5 is connected along its perimeter to the supportingstructure 2 and, in one embodiment, is of semiconductor material and iscircular. For instance, the membrane 5 has a diameter between about 600μm and about 800 μm and a thickness between about 0.5 μm and about 15μm.

Opposite faces of the membrane 5 are coated with respectivesilicon-oxide protective layers 13, 14. In the example of FIGS. 1 and 2, the membrane 5 is continuous and has a circular shape. However, insome embodiments (not illustrated), the membrane 5 may be perforated andhave an arbitrary shape, according to the design preferences.

In addition, the membrane piezoelectric actuator 8 is set at the centeron the membrane 5 and comprises a plate of piezoelectric materialbetween a first membrane electrode 16 and a second membrane electrode17, which are coupled to the driving device 12 for alternativelysupplying membrane-actuation signals SM and receiving reception signalsSR. The piezoelectric material may, by way of non-limiting example, bePZT (Lead Zirconate Titanate). The membrane 5, the membranepiezoelectric actuator 8, and the membrane electrodes 16, 17 are coatedwith a passivation layer, for example a multilayer of USG (undopedsilicate glass) 18 and silicon nitride 19.

The cantilever damper 7 comprises a bracket 20 of an annular shape,fixed to the supporting structure 2 around the perimeter of the membrane5 and extending towards the inside of the membrane 5 at a distancetherefrom. More precisely, an anchorage region 21, from which thebracket 20 protrudes, is joined to the supporting structure 2 by anadhesion layer 23 around the membrane 5. The bracket 20 projects, forexample, by an amount of between about 100 μm and about 200 μm from theanchorage region 21 and has a thickness between about 0.5 μm and about 5μm. A stopper element 25, which also has an annular shape, extends fromthe radially inner edge of the bracket 20 in the direction toward themembrane 5. In one embodiment, the anchorage region 21 and the stopperelement 25 have the same structure; for example, they are of epitaxialsilicon coated with silicon oxide on the side facing the membrane 5.When the membrane 5 is at rest and the damper piezoelectric actuator 10is not activated, the stopper element 25 is located, for example, at adistance of approximately 1 μm from the membrane 5.

The damper piezoelectric actuator 10 comprises a continuous annularregion of piezoelectric material, for example PZT, set on the cantileverdamper 7 along an inner perimetral edge thereof, in particular on a faceof the bracket 20 opposite to the membrane 5 and comprised between afirst damper electrode 26 and a second damper electrode 27. Forinstance, the annular region of piezoelectric material has a thickness(perpendicular to the membrane 5) between about 0.5 μm and about 3 μmand a width between about 80 μm and about 1000 μm, in a directionparallel to the membrane 5. In the case illustrated, where the annularregion of piezoelectric material has an annular shape, the width is thedifference between the outer radius and the inner radius.

A passivation layer, for example a multilayer of USG 28 and siliconnitride 29 coats the damper piezoelectric actuator 10, the damperelectrodes 26, 27, and the remaining portion of the bracket 20.

The damper piezoelectric actuator 10 is configured to bend thecantilever damper 7, in particular the bracket 20, towards the membrane5 in response to a damper-actuation signal SD, as illustrated in FIG. 4. In one embodiment, the damper piezoelectric actuator 7 extends along aradially inner margin of the bracket 20. Bending of the bracket 20brings the stopper element 25 into contact with the membrane 5 and inpractice causes an increase of the damping coefficient of theoscillating system. Activation of the damper piezoelectric actuator 7thus reduces the time of suppression of the oscillations of themembrane.

FIGS. 5 a-5 d show the comparison between the behaviour of the membrane5 with and without intervention of the damper piezoelectric actuator 7.More precisely, FIGS. 5 a and 5 b show the amplitude of oscillation ofthe membrane 5, whereas FIGS. 5 c and 5 d show the pressure in air at areference distance from the plane of the membrane 5 at rest. The graphsof FIGS. 5 a and 5 c are obtained without activation of the membranepiezoelectric actuator 7. The graphs of FIGS. 5 b and 5 d are obtainedwith the membrane piezoelectric actuator 7 activated and clearly showthe effect thereof in terms of reduction of the oscillation dampingtime.

The driving device 12 is configured to apply membrane-actuation signalsSM to the membrane piezoelectric actuator 8, in a transmitting mode, andto receive and amplify reception signals SR generated by the membranepiezoelectric transducer 8 as a result of the oscillations of themembrane 5. The driving device 12 switches between the transmittingmode, where the membrane piezoelectric transducer 12 is used fortransmitting a packet of pressure waves, and the receiving mode, wherethe membrane piezoelectric transducer 8 is used for detecting echoes dueto reflection of the pressure waves transmitted onto obstacles set alongthe propagation path.

Furthermore, the driving device 12 is configured to applydamper-actuation signals SD to the damper piezoelectric actuator 10. Inresponse to the damper-actuation signals SD, the damper piezoelectricactuator 10 causes bending of the bracket 20, and the stopper element 25comes into contact with the membrane 5 and causes a faster damping ofthe oscillations.

The membrane-actuation signals SM and the damper-actuation signals SDmay be synchronised, as indicated in FIGS. 6 a and 6 b . In particular,a membrane-actuation signal SM (FIG. 6 a ) increases according to a rampfrom a minimum membrane value VMINM to a maximum membrane value VMAXMand, after a settling interval, oscillates between the minimum membranevalue VMINM and the maximum membrane value VMAXM for a number ofprogrammed cycles. The damper-actuation signal SD (FIG. 6 b ) increasesaccording to a ramp from a minimum damper value VMIND to a maximumdamper value VMAXD so as to bring the stopper element 25 gradually intocontact with the membrane 5 immediately after the number of programmedcycles of the membrane-actuation signal SM has been completed. Inparticular, contact occurs before the transient of natural damping ofthe oscillations of the membrane 5 is over (i.e., without interventionof the damper piezoelectric actuator 10). Start of the ramp is selectedso that the contact of the stopper element 25 with the membrane 5 occursat a desired instant. When the transient of damping of the oscillationsof the membrane 5 is over, the damper-actuation signal SD decreasesaccording to a ramp from the maximum damper value VMAXD to the minimumdamper value VMIND so as to interrupt gradually the contact between thestopper element 25 and the membrane 5. Gradual contact (primarily) andgradual detachment (secondarily) between the stopper element 25 and themembrane 5 prevent perturbation of the state of the membrane 5 itself,which is soon ready for the receiving mode.

In the embodiment of FIG. 7 , where parts that are the same as the onesalready illustrated are designated by the same reference numbers, amicroelectromechanical membrane transducer 100 comprises a discontinuousdamper piezoelectric actuator 110, defined by a plurality of distinctregions of piezoelectric material, each extending in the form of anannular sector along a respective portion of the inner perimetral edgeof the cantilever damper 107. The portions of the damper piezoelectricactuator 110 are concentric and have the same radius and, moreover, areindividually connected to the driving device 12. In one embodiment, theportions of the damper piezoelectric actuator 110 can be controlledindependently by the driving device 12.

With reference to FIG. 8 , in a microelectromechanical membranetransducer 200 according to one embodiment of the disclosure, thecantilever damper 207 comprises a plurality of brackets 220, whichextend from respective anchorage regions 221 towards the membrane 5 andare shaped like annular sectors along respective portions of theperimeter of the membrane 5. The brackets 220 are separated from oneanother by gaps 221. Each bracket 220 is provided with a respectivedamper piezoelectric actuator 210, which comprises a region ofpiezoelectric material extending along a respective annular sector. Thedamper piezoelectric actuators 210 can be controlled separately by thedriving device 12.

FIG. 9 shows a microelectromechanical membrane transducer 300 accordingto an embodiment of the disclosure. The microelectromechanical membranetransducer 300 comprises a perforated membrane 305 and a membranepiezoelectric transducer 308 having a star shape, with radial arms. Infurther embodiments (not illustrated), the damper piezoelectric actuatormay comprise concentric regions of piezoelectric material having acircular shape or the shape of an annular sector.

A process for manufacturing the membrane piezoelectric transducer 1 isschematically illustrated in FIGS. 10-14 .

A first semiconductor wafer 500 (FIG. 10 ) comprises a substrate 501 ofmonocrystalline silicon, on which the protective layer 13, the structureof the membrane 5 (for example, via growth of a pseudo-epitaxial layerof monocrystalline silicon, the membrane 5 will be released thereafter)and the protective layer 14 are formed in succession. Next, a stackformed by a layer of platinum, a layer of piezoelectric material (forexample PZT), and a layer of titanium and tungsten alloy (TiW) isdeposited on the protective layer 14 and defined to form the firstmembrane electrode 16, the membrane piezoelectric actuator 8, and thesecond membrane electrode 17. Then, the passivation layer is provided bydepositing the layer of USG 18 and the layer of silicon nitride 19.

A second semiconductor wafer 510 (FIG. 11 ) comprises a substrate 511 ofmonocrystalline silicon. A silicon-oxide layer is formed on thesubstrate 511 and defined so as to leave anchorage regions 21 a andstopper regions 25 a where the anchorage region 21 and the stopperelement 25 will subsequently be formed. An epitaxial layer 513 is thengrown, and a silicon-oxide layer is formed on the epitaxial layer 513and subsequently defined for providing the bracket 20, with a centralopening 515 corresponding to the membrane 5.

Next, a stack formed by a layer of platinum, a layer of piezoelectricmaterial (for example, PZT) and a layer of titanium and tungsten alloy(TiW) is deposited on the protective layer 14 and defined to form thefirst damper electrode 26, the damper piezoelectric actuator 10, and thesecond damper electrode 27.

Then, the passivation layer is obtained by depositing the USG layer 28and the silicon-nitride layer 29 so as to coat the damper piezoelectricactuator 10, the damper electrodes 26, 27, and the remaining portion ofthe bracket 20. Within the bracket 20, the passivation layer isselectively removed.

The second semiconductor wafer 510 is then joined to an auxiliarysupporting wafer 515 by an adhesion layer 516, and the substrate 511 isthinned out to a controlled thickness.

Next, the substrate 511 and the epitaxial layer 513 are etched with ananisotropic dry etch. In this step, the substrate 511 is removedcompletely, whereas the epitaxial layer 513 is partially protected bythe anchorage regions 21 a and the stopper regions 25 a. The anchorageregion 21 and the stopper element 25 are thus formed.

The first semiconductor wafer 500 and the second semiconductor wafer 510are joined together by the adhesion layer 23, are flipped over, and thesubstrate 501 of the first semiconductor wafer 500 is anisotropicallyetched with a dry etch, until one side of the membrane 5 is released.

Finally, the second semiconductor wafer 510 is separated from theauxiliary supporting wafer 515 by removing the adhesion layer 516. Thestructure of the membrane piezoelectric transducer 1 illustrated in FIG.2 is thus obtained.

Finally, it is evident that modifications and variations may be made tothe microelectromechanical membrane transducer, to the control method,and to the manufacturing process described herein, without therebydeparting from the scope of the present disclosure.

A microelectromechanical membrane transducer may be summarized asincluding a supporting structure (2); a cavity (3) formed in thesupporting structure (2); a membrane (5; 305) coupled to the supportingstructure (2) so as to cover the cavity (3) on one side; a cantileverdamper (7), which is fixed to the supporting structure (2) around theperimeter of the membrane (5; 305) and extends towards the inside of themembrane (5; 305) at a distance from the membrane (5; 305); and a damperpiezoelectric actuator (10; 110; 210), arranged on the cantilever damper(7) and configured so as to bend the cantilever damper (7) towards themembrane (5; 305) in response to an electrical actuation signal (SD).

The damper piezoelectric actuator (10; 110; 210) may extend along aninner perimetral edge of the cantilever damper (7).

The damper piezoelectric actuator (10) may be continuous along theentire inner perimetral edge of the cantilever damper (7).

The damper piezoelectric actuator (110; 210) may include a plurality ofdistinct regions of piezoelectric material, each arranged along arespective portion of the perimeter of the inner perimetral edge of thecantilever damper (7).

The cantilever damper (7) may include a bracket (5; 305) fixed to thesupporting structure (2) along a perimeter of the membrane (5; 305) andwherein the damper piezoelectric actuator (110; 210) is set on a face ofthe bracket (5; 305).

The cantilever damper (7) may include a stopper element (25) extendingfrom an inner edge of the bracket (20) towards the membrane (5) andwherein the damper piezoelectric actuator (110) is set on a face of thebracket (5; 305) opposite to the membrane (5).

The cantilever damper (7) may include an anchorage region (21), joinedto the supporting structure (2) by an adhesion layer (23) around themembrane (5) and wherein the bracket (20) protrudes from the anchorageregion (21).

The cantilever damper (207) may include a plurality of brackets (220),which extend from respective anchorage regions (221) towards themembrane (5) along respective portions of the perimeter of the membrane(5) and wherein each bracket (220) is provided with a respective damperpiezoelectric actuator (210).

The transducer may include a driving device (12) configured to apply theelectrical actuation signals (SD) to the damper piezoelectric actuator(10; 110; 210).

Each portion of the damper piezoelectric actuator (110) may beindependently controlled by the driving device (12).

Each damper piezoelectric actuator (210) may be independently controlledby the driving device (12).

The transducer may include a membrane piezoelectric actuator (8; 308) ona face of the membrane (5; 30), wherein the driving device (12) isconfigured to alternatively apply membrane-actuation signals (SM) to themembrane piezoelectric actuator (8; 308) in a transmitting mode and toreceive reception signals (SR) from the membrane piezoelectric actuator(8; 308) in a receiving mode.

The driving device (12) may be configured to cause oscillation of themembrane-actuation signal (SM) between a first membrane value (VMINM)and a second membrane value (VMAXM) for a programmed number of cyclesand to modify according to a ramp the damper-actuation signal (SD)between a first damper value (VMIND) and a second damper value (VMAXD)so as to bring the cantilever damper (7) gradually into contact with themembrane (5; 305) after the programmed number of cycles of themembrane-actuation signal (SM) has been completed.

A method for controlling a microelectromechanical membrane transducermay be summarized as including forcing a programmed number of cycles ofoscillation of the membrane (5; 305); and bringing the cantilever damper(7) into contact with the membrane (5; 305) after the programmed numberof cycles of oscillation of the membrane (5; 305) has been completed andbefore a transient of damping of the oscillations of the membrane (5;305) is over.

A process for manufacturing a microelectromechanical membrane transducermay be summarized as including forming a structure of a membrane (5) ona first substrate (501) of a first semiconductor wafer (500); forming acantilever damper (7) on a second substrate (511) of a secondsemiconductor wafer (510), with a central opening (515) in a positioncorresponding to the structure of the membrane (5); joining the firstsemiconductor wafer (500) and the second semiconductor wafer (510) withthe structure of the membrane (5) facing the cantilever-damper structure(7); and releasing the membrane (5; 305), wherein forming the cantileverdamper (7) comprises forming a damper piezoelectric actuator (10; 110;210) configured so as to bend the cantilever damper (7) towards themembrane (5; 305) in response to an electrical actuation signal (SD).

The various embodiments described above can be combined to providefurther embodiments. Aspects of the embodiments can be modified toprovide yet further embodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. A process for manufacturing a microelectromechanical membranetransducer, comprising: forming a structure of a membrane on a firstsubstrate of a first semiconductor wafer; forming a cantilever damper ona second substrate of a second semiconductor wafer; joining the firstsemiconductor wafer and the second semiconductor wafer with thestructure of the membrane facing the cantilever-damper structure with acentral opening of the cantilever damper in a position corresponding tothe structure of the membrane; and releasing the membrane by forming acavity in the first substrate below the structure of the membrane;wherein the forming the cantilever damper comprises forming a damperpiezoelectric actuator configured to bend the cantilever damper towardsthe membrane in response to an electrical actuation signal.
 2. Themethod of claim 1, further comprising forming a stack of a layer ofplatinum, a layer of piezoelectric material, and a layer of titanium andtungsten alloy on the structure of the membrane.
 3. The process of claim1, wherein forming the cantilever damper further includes: forming ananchorage region; forming a first end of a bracket coupled to theanchorage region and the bracket extending laterally from the anchorageregion; and forming a stopper element coupled to a second end of thebracket opposite to the first end of the bracket and spaced laterallyfrom the anchorage region.
 4. The process of claim 3, wherein joiningthe first semiconductor wafer and the second semiconductor wafer furtherincludes: joining the first semiconductor wafer and the secondsemiconductor wafer at the anchorage region; and defining a spacebetween the stopper element of the cantilever damper and the structureof the membrane.
 5. The process of claim 4, wherein the space provides adegree of freedom to allow the damper piezoelectric actuator to bend thecantilever damper towards the membrane in response to an electricalactuation signal.
 6. The process of claim 1, wherein joining the firstsemiconductor wafer and the second semiconductor wafer further includes:coupling the second semiconductor wafer to an adhesion layer on asupporting wafer; and removing the second substrate from the secondsemiconductor wafer.
 7. The process of claim 6, wherein coupling thesecond semiconductor wafer to the adhesion layer on the support waferincludes coupling the cantilever damper on the second substrate of thesecond semiconductor wafer to the adhesion layer on the supportingwafer.
 8. A process for manufacturing a microelectromechanical membranetransducer, comprising: forming a structure of a membrane on a firstsubstrate of a first semiconductor wafer; forming a cantilever damper ona second substrate of a second semiconductor wafer; coupling thecantilever damper and the second substrate to an adhesion layer on asupporting wafer; removing the second substrate from the cantileverdamper and leaving the cantilever damper coupled to the adhesion layer;defining a space between a stopper element of the cantilever damper andthe structure of the membrane by coupling the structure of the membraneto an anchorage region of the cantilever damper; and releasing themembrane by forming an opening overlapping the membrane by removing aportion of the first substrate.
 9. The process of claim 8, furthercomprising: forming stopper regions on the second substrate of thesecond semiconductor wafer; forming an epitaxial layer on the secondsubstrate of the second semiconductor wafer and on the stopper regionson the second substrate of the semiconductor wafer; and forming theanchorage element and the stopper element of the cantilever damper byetching the epitaxial layer on the first substrate.
 10. The process ofclaim 9, wherein forming the anchorage element and the stopper elementby etching the epitaxial layer on the first substrate further includesremoving respective portions of the epitaxial layer spaced laterallyfrom the stopper regions.
 11. The process of claim 10, wherein thestopper regions prevent etching of respective portions of the epitaxiallayer aligned with and overlapped by the stopper regions.
 12. Theprocess of claim 8, wherein forming the structure of the membrane on thefirst substrate of the first semiconductor wafer further includesforming one or more protective layers, one or more conductive layers onthe first substrate of the first semiconductor wafer, and forming atleast one piezoelectric layer on the first substrate of the firstsemiconductor wafer.
 13. The process of claim 12, wherein forming theone or more conductive layers includes: forming a first electrode on afirst side of the at least one piezoelectric layer; and forming a secondelectrode on a second side of the at least one piezoelectric layeropposite to the first side of the at least one piezoelectric layer. 14.The process of claim 13, wherein forming the first electrode, formingthe second electrode, and forming the at least one piezoelectric layerdefines a piezoelectric actuator of the structure of the membrane. 15.The process of claim 8, further comprising releasing the membrane byforming an opening overlapping the membrane by removing a portion of thefirst substrate.
 16. A process for manufacturing amicroelectromechanical membrane transducer, comprising: forming astructure of a membrane on a first substrate of a first semiconductorwafer; forming an annular cantilever damper on a second substrate of asecond semiconductor wafer; coupling the annular cantilever damper andthe second substrate to an adhesion layer on a supporting wafer;removing the second substrate from the annular cantilever damper andleaving the annular cantilever damper coupled to the adhesion layer;defining a space between an annular stopper element of the annularcantilever damper and the structure of the membrane by coupling thestructure of the membrane to an annular anchorage region of the annularcantilever damper; releasing the membrane by forming an openingoverlapping the membrane by removing a portion of the first substrate.17. The process of claim 16, further comprising: forming annular stopperregions on the second substrate of the second semiconductor wafer;forming an epitaxial layer on the second substrate of the secondsemiconductor wafer and on the annular stopper regions on the secondsubstrate of the semiconductor wafer; forming the anchorage element andthe stopper element of the cantilever damper by etching the epitaxiallayer on the first substrate.
 18. The process of claim 17, wherein theannular stopper regions include an inner annular stopper region and anouter annular stopper region that surrounds the inner annular stopperregion.
 19. The process of claim 18, wherein the annular stopper regionprevent etching of respective portions of the epitaxial layer alignedwith and overlapped by the stopper regions.
 20. The process of claim 18,wherein the inner annular stopper region corresponds to the annularstopper element of the annular cantilever damper and the outer annularstopper region corresponds to the annular anchorage region of theannular cantilever damper.